Different structure and chemical composition of the insulating materials of medium voltage XLPE cables lead to various dielectric responses of new (unaged) cables and, therefore, can lead to problems in the diagnosis in the case of water tree aged medium voltage cables. With the aim of gaining a better understanding of these problems from a technical point of view, the results of dielectric response measurements in the time as well as in the frequency domain on different new XLPE homo- and copolymer insulated medium voltage cables and on a pressed sandwich-type test specimen before and after thermal treatment are presented and discussed. The investigations show that XLPE copolymer materials give higher dielectric responses than XLPE homopolymer materials. One of the reasons is that XLPE copolymer materials have a higher amount of polar, volatile elements (e.g. crosslinking byproducts), which lead especially to a higher voltage dependent conductivity